Field of the Invention
The invention relates primarily to a monitoring circuit for an electromagnetic clutch with regard to engaged and disengaged states thereof. The invention may furthermore be utilized in related electromagnetically actuated devices, such as control elements, servo-actuators, brakes, valves, and the like.
With machines, particularly with printing presses having complex, electrical and electronic controls, respectively, it may be necessary to obtain switching signals which provide information regarding the engaged and disengaged states of an electromagnetically operated clutch. Various possible solutions have become known heretofore for this purpose. One heretofore proposed solution calls for monitoring the position of that part of the clutch which is moved as a result of the respective engagement and disengagement. In the disengaged state, that part assumes a first position and, after a voltage has been injected in the electromagnet provided for the operation of the clutch, that part assumes a second position. The distance covered by that part from the disengaged to the engaged state is of the order of magnitude of some tenths of a millimeter to a few millimeters and can be measured with conventional position sensors, preferably of electromechanical, opto-electronic, inductive or capacitive types. In the simplest case, a microswitch may be installed permanently on the frame, and may be actuated when the clutch is in the properly and precisely engaged state.
German Published Document 31 30 259 A1 discloses a circuit configuration for a power-shift transmission wherein hydraulically operated disconnect-type clutches are provided, a displacement- or pressure-measuring system being installed in order to determine the position of the clutch. Furthermore, this circuit configuration is provided with tachometers on the input and output sides, the tachometers being connected to a control unit including a rotational-speed control loop.
A further heretofore proposed solution calls for installing a device for displacement or angle measurement as an indicator of the engaged state of a clutch, this measurement device being associated with the machine part driven by the clutch. A transmission of signals from the displacement- and angle-measuring devices, respectively, can be taken as an indication that the clutch is engage.
A disadvantage of these heretofore proposed solutions is that they each require additional, costly devices for measuring displacement, angle and rotational speed, respectively, which require additional installation space, are complicated to install and, because of their manner of construction and their complexity, adversely affect reliability with regard to the monitoring of an electromagnetic clutch. Moreover, with such proposed solutions, which are partly realized or implemented with mechanical means, one must expect that, during rough machine operation, the accuracy with which the position of the moved clutch and machine parts is determined will be lost due to mechanical and other ambient influences.
With some machines, particularly with printing presses, it may be necessary, in addition to monitoring the engaged and disengaged states, also to monitor the precise angular position of the connecting or coupling parts of a rotating clutch. In the case of toothed clutches, for example, it is possible that, during engagement and as a result of operational disturbances, respectively, the teeth may not be seated in the mating numbers provided therefor, or that the teeth do not lock in position precisely and sit in an undefined position on the side of a tooth.
Monitoring devices required for this purpose are likewise disposed as additional components on the clutch and on the driving and/or driven machine elements, respectively, and have the same disadvantages as described heretofore.
In the case of simple monitoring arrangements for an electromagnetically operated clutch, it is possible, by measuring the DC-current consumption of the electromagnet, to obtain a signal for the "clutch engaged" state. Thus, it is possible, for example, in the connection or terminal line to the electromagnet, to connect an ohmic resistance in series with the electromagnet, the voltage drop across the resistance being evaluated. If the voltage drop exceeds a given threshold value, it is possible then, at the output of a comparator connected to the resistance, to provide a signal pulse edge or slope which can be taken as an indication that the clutch is in the engaged state.
With this heretofore proposed solution, it is assumed that, when in electromagnetic clutch is supplied with voltage and current, respectively, it assumes its intended position, i.e. a toothed clutch, for example, locks precisely in place with respect to the axial position and angular position of the machine elements which are to be coupled. A disadvantage thereof is that, due to the measurement of the DC-current susceptibility or consumption of the electromagnet, it is not possible to detect minor deviations from the precisely engaged state, because there is only an insignificant change in the DC current due to the saturation of the electromagnet in these ranges.
It is accordingly an object of the invention to provide a circuit configuration for monitoring an electromagnetically actuated clutch which permits at low cost and with relatively little outlay of materials, a reliable monitoring of the engaged and disengaged states of the clutch and of the angular position of the connecting or coupling machine elements.
With the foregoing and other objects in view there is provided, in accordance with the invention, a circuit configuration for monitoring an electromagnetically actuated device, such as an electromagnetic clutch, having an electromagnet with a winding, comprising a switchable power source connected to the winding of the electromagnet, the power source being means for supplying a voltage formed of a direct voltage part and a periodic alternating voltage part superimposed on the direct voltage part, means for issuing engage and disengage signals for the device and for providing the direct voltage part at two defined levels in dependence on the engage and disengage signals, a precision resistor connected in series with the winding of the electromagnet, a voltage sensor connected at a node between the winding and the precision resistor, and an evaluation circuit connected to the voltage sensor for evaluating the alternating voltage part acting on the precision resistor.
In accordance with an added feature of the invention, circuit configuration includes an alternating voltage source supplying the alternating voltage part with a given period of oscillation to the power source, the voltage sensor being an alternating voltage amplifier, and the evaluation circuit including a comparator having first and second inputs and an output, the first input being connected to an output of the alternating voltage amplifier, a reference voltage source connected to the second input, the reference voltage source providing two defined reference voltage levels in dependence on the engage and disengage signals, a flip flop having a set input connected to the output of the comparator and a reset input connected to the alternating voltage source, a one-shot connected with an output of the flip flop and having an ON-time greater than the given period of oscillation, and an exclusive-NOR member connected with an output of the one-shot, the exclusive-NOR member having an input connected to the issuing means for receiving the engage and disengage signals.
In accordance with an additional feature of the invention, the means for issuing the engage and disengage signals are a switch by which the device is turned on or off.
In accordance with a further feature of the invention, the voltage sensor is a direct voltage amplifier, and the evaluation circuit includes an alternating voltage amplifier connected downstream of the direct voltage amplifier, a rectifier circuit connected downstream of the alternating voltage amplifier, two sample-and-hold circuits connected downstream of the rectifier circuit, the sample-and-hold circuits having pulse inputs, first and second pulse forming circuit means respectively connected between the alternating voltage source and the respective pulse inputs of the sample-and-hold circuits, an adder connected downstream of the sample-and-hold circuits, and a voltage measurement circuit connected to an output of the adder.
In accordance with another feature of the invention, the first pulse forming circuit includes a high-pass and a non-inverting Schmitt-trigger, and the second pulse forming circuit includes a high-pass and an inverting Schmitt-trigger.
In accordance with yet another feature of the invention, the voltage measurement circuit includes a comparator having a first input connected with an output of the adder, and a voltage divider connecting a second input of the comparator to the direct voltage source.
In accordance with yet an additional feature of the invention, the circuit configuration includes an AND-member having a first input connected to an output of the comparator and a second input, and a window comparator having an output connected to the second input of the comparator.
With the objects of the invention in view there is also provided, in accordance with added features of the invention, a circuit configuration for monitoring an electromagnetic device, such as a clutch, having an electromagnet with a winding, comprising:
direct voltage means for supplying a direct voltage at two distinctly different voltage levels;
alternating voltage means for supplying a periodically alternating voltage with a given period of oscillation;
power source means for superimposing the direct and alternating voltages on one another for forming a composite voltage having a direct voltage part and an alternating voltage part, and for switching the direct voltage part to a relatively low level upon issuance of a disengage signal for the device, and for switching the direct voltage part to a relatively higher level upon issuance of an engage signal for the device;
the power source means injecting a voltage in the winding of the electromagnet independently of the engage or disengage signals;
voltage evaluation circuit means connected to the winding of the electromagnet for evaluating a voltage at the winding and determining a state of the electromagnetically actuated device.
In accordance with a concomitant feature of the invention, the voltage evaluation circuit means include voltage sensor means in the form of an alternating voltage amplifier, a comparator having a first input connected to an output of the alternating voltage amplifier, a reference voltage source connected to a second input of the comparator, the reference voltage source providing two defined reference voltage levels in dependence o the engage and disengage signals, a flip flop having a set input connected to an output of the comparator and a reset input connected to the alternating voltage means, a one-shot connected with an output of the flip flop and having an ON-time greater than the given period of oscillation, and an exclusive-NOR gate connected with an output of the one-shot and with the means for issuing the engage and disengage signals.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
According to the invention a periodic alternating current is superimposed on the trigger current of the electromagnetic device, for example of the magnetic clutch. The voltage drop of the alternating voltage across a resistance which is connected in series with the winding is evaluated in a circuit configuration for a signal for monitoring the engaged and disengaged states of the electromagnetic device.
It is advantageous in the signal evaluation, if a periodic rectangular pulse train signal with a duty cycle ratio of greater or equal 50% is used in the superposition.
Although the invention is illustrated and described herein as embodied in a circuit configuration for monitoring an electromagnetically actuated device, in particular an electromagnetic clutch, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of the specific embodiment when read in connection with the accompanying drawings.